The invention relates to instruments for measuring the atmospheric transmission along a measurement path and, in particular for measuring the atmospheric transmission in the 3-5micrometer infrared spectral region between a flying aircraft and a ground-based, shipborne, or airborne infrared detection station.
Transmissometers are devices used to measure the transmissivity along a specific path. The basic components of a transmissometer are a transmitter and a receiver. The transmitter transmits a calibrated beam of electromagnetic radiation to the receiver and the output of the receiver is monitored to determine the degree of attenuation of the radiation beam.
Transmissometers have been used in various applications such as monitoring smoke stack emissions, calculating mass concentrations, determining the visibility at airports, and determining infrared transmissions along various paths, etc. Infrared transmissometers were normally limited to performing such measurements along horizontal or slant paths, where the transmitter and receiver are stationary in position. They were incapable of accurately measuring the infrared atmospheric transmission along a path between a flying aircraft and a detection station, a measurement which is of paramount importance to the U.S. Government.
It is also very important to perform such measurements simultaneously with other tests being conducted on the test target aircraft where the system under test is an Infrared Search and Track (IRST) system, a Forward Looking Infrared (FLIR), or an infrared missile seeker. The importance of the subject transmissometer derives from the fact that when newly developed infrared detection systems, such as IRSTs, FLIRs or infrared missile seekers, are being tested against moving targets, the infrared transmission of the atmospheric path between the system under test and the test targets must be accurately measured. Only then can the performance of the device being tested be fully quantified.